Dolastatin 15 is a substance which is described in U.S. Pat. No. 4,879,278 and has the following formula. ##STR2##
Dolastatin 15 is attracting great interest because of its great efficacy against various tumors. Its isolation from the lumpfish which is difficult to obtain is lengthy and time-consuming, and the process provides the active substance in moderate yield and poorly reproducible quality. In order to make the active substance available in gram quantities for animal experiments, Pettit et al. (J. Am. Chem. Soc. 113 1991! 6692-6693) have developed a synthetic method. The central intermediate in this is the tetrapeptide of the formula Ia ##STR3##
WO 93/23424 describes antineoplastic active substances whose effect exceeds that of dolastatin 15. Many active substances in WO 93/23424 can be prepared from tetrapeptides of the formula I ##STR4## where R.sup.1 -R.sup.4 are C.sub.1 -C.sub.6 -alkyl groups.
In order to make the peptides of WO 93/23424 and dolastatin 15 available in sufficient quantity for clinical tests it was necessary to find a process for preparing the tetrapeptides I which can be implemented industrially.
The solid-phase synthesis described in WO 93/23424 is unsuitable for preparing large amounts of product. It provides impure product which requires laborious chromatographic purification.
Pettit et al. (J. Am. Chem. Soc. 113 1991! 6692-6693) describe an elegant laboratory synthesis for Ia. This entails the tripeptide Val--MeVal--Pro--OMe being reacted with ##STR5## (Short name: Me.sub.2 Val--OPfp) to give the tetrapeptide Ia.
The synthesis of Me.sub.2 Val--OPfp takes place according to Pettit et al. (U.S. Pat. 4,978,744) as follows:
Initially valine is dimethylated on the nitrogen. The resulting dimethylvaline must be reacted with a condensing agent and pentafluorophenol (HO-Pfp). ##STR6##
This process is very troublesome owing to the large number of reaction steps required. An additional factor is that pentafluorophenol is a very costly reagent which is unavailable in sufficient quantity for industrial syntheses. In addition, its use leads to fluorine-containing wastes which can be disposed of only with difficulty and possibly with formation of dioxins.
The use of condensing agents in the synthesis also leads to problems. Thus, it is known that handling dicyclohexylcarbodiimide may lead to sensitization and extremely severe allergic reactions. Workup of the batches produces the corresponding urea which can be removed from the product only laboriously and often only incompletely. Carbodiimides which, like N-ethyl-N'-dimethylaminopropylcarbodiimide, react to form water-soluble ureas are extremely costly and are not available in industrial quantities.
Replacement of pentafluorophenol/carbodiimide by pentafluorophenyl trifluoroacetate or its carbonates, eg. pentafluorophenyl 1,2,2,2-tetrachloroethyl carbonate (J. Org. Chem. 52 1987! 2364) also provides no advantages.
Racemization occurs in active ester formation and peptide coupling and represents a serious problem in this method (see J. Jones: The Chemical Synthesis of Peptides, Oxford 1991, page 57).
Another published method for linking dimethylvaline to other amino acids (T. Shioiri et al.: Tetrahedron 49 1993! 1913-1924), which uses diethylphosphoryl cyanide (DEPC) as coupling reagent, likewise involves serious disadvantages: DEPC is very costly, not obtainable in large quantity, corrosive and very toxic. The cyanide-containing mother liquors and washings must be disposed of as special waste.